The present invention relates to unstable optical cavities for laser beams, used especially in the field of laser cavities and optical parametrical oscillators.
These cavities are called unstable because a light ray circulating in these cavities moves away from the optical axis as and when it is being propagated. The external part of the beam circulating in the cavity is deliberately transmitted out of the cavity by an output mirror designed for this purpose. This external part is the useful beam.
The mirrors used in prior art optical cavities have limited resistance to flux. Indeed, the light flux that flows in these cavities tends to cause deterioration in the multidielectric treatment of the mirrors used for changes in direction.
The present invention is aimed at overcoming this drawback.
To obtain this goal, two methods were possible: either to find modes of multidielectric treatment that are more resistant or else to remove the need for such treatment. It is a second method that was explored in the context of the invention.
According to the invention, there is proposed an unstable optical cavity for laser beams, in which a beam is created. This cavity has an optical path known as a ring path with n changes in direction, where n is an integer greater than 2, to form the ring and comprising, in the optical path, an optical magnification set, n deflection means to make the beam created in the cavity undergo the n changes in direction, and an output, wherein the deflection means are n prisms each of which has an interface, called a reflecting interface, of the air-glass type that provides one of the changes in direction by total reflection and wherein nxe2x88x921 prisms, called large prisms, have their reflecting interface with a sufficient size for the reflection, in the cavity, of the totality of the beam while one of the prisms, called an output prism, is positioned at the output and has its reflected interface with an insufficient size for the reflection, in the cavity, of the totality of the beam created in the cavity.